DOWN-HOLE TRIAXIAL TEST TO MEASURE AVERAGE STRESS - STRAIN RELATIONSHIP OF ROCK MASS

KAZUO TANI, TAKAJI NOZAKI, SUSUMU KANEKO,
YOSHINORI TOYO-OKA and HIDEO TACHIKAWA

ABSTRACT: A new field test method was proposed for the purpose of measuring average stress-strain relationships of rock masses. The test is conducted on a hollow cylindrical specimen prepared at the bottom of a drill-hole. Average axial as well as lateral strains can be measured in a center hole and an outer slit by a novel technique of instrumentation for cavity deformation.
A set of test equipment for this test method was developed at Central Research Institute of Electric Power Industry, CRIEPI, in Japan for prototype tests. Specimens of 400 mm in outer diameter and 1050 mm in height can be sheared under confining pressures as high as 5.0MPa. Accuracy of strain measurements was designed to be of the order of 10^-4.
Trial series of tests were carried out at the site of rhyolitic tuffacious rock formation. The results, similar to conventional laboratory triaxial tests, proved that the proposed test method was successful to measure average stress-strain relationships of large rock specimens. Considering the significant size of the specimens, the volume of rock involved in the tests is of the same order or greater than those in conventional field rock tests, such as plate load tests and rock shear tests. As a consequence, the test results are considered as representative in evaluation of strength and deformation characteristics of rock masses but not rock cores.

Key words: deformation, field test, rock mass, shear strength, test equipment, test procedure, triaxial test (IGC: C3/F6)